Sept. 13 (UPI) -- Hundreds of protesters gathered outside a climate change summit in California Thursday, calling for elected officials to do more to combat climate change and pollution.

Gov. Jerry Brown, who organized the Global Climate Action Summit, was the target of some of the criticism brought by the demonstrators as some held signs warning this was his last chance to choose between "fossil fuel or our future," The Los Angeles Times reported.

"We have a plan to phase out oil, to bring in total clean technology," Brown said. "If they have a better one, let me know."

Protesters called for Brown to create new protection zones around oil and gas production sites and questioned his decisions to approve new oil drilling permits.

The crowds at one point also blocked an entrance to the Moscone Center, where the summit was being held, and called for some of the 4,000 elected officials attending the three-day conference to consider alternatives to market-based climate change solutions, The San Francisco Chronicle reported.

Some indigenous people traveled from outside the state to call attention to the dangers of pollution.

Inside the summit, Brown and former New York Mayor Michael Bloomberg detailed goals to reduce emissions by shifting drivers toward electric vehicles and closing coal plants at a more rapid pace.

"Climate change is still not at the top of the political issues I don't think in any country," Brown said. "But we are putting it there. And we have to constantly raise our ambition. The threat of climate change is as real as any we deal with."

Bloomberg noted that the U.S. reduced carbon emissions more than any other large nation since 2000, as 27 cities announced they had reached their goals of decreasing their emissions to prevent catastrophic global warming in 2020.

On Monday, Brown signed a bill aiming to eliminate fossil fuel use for electricity in the state by 2045, in response to U.S. withdrawal from the Paris climate accord.